Cold cathode and method of preparing same



March 17, 1953 J. MORRlSON, JR 2,631,945

cow CATHODE AND METHOD OF PREPARING SAME FiledNov. 19, 1949 FIG 2FORM/N6 AND CLEANING THE NICKEL BLANK HEAT TREATING THE BLANK IN WET HAT [200C FOR I5 MIN.

CALORIZING THE BACK OF THE BLANK HEAT TREA TING THE BLANK 11v 0m H2 AT925'0 FOR 15 MIN.

oxmIzI/vc THE BLANK IN 95% N 5% 0 AT 700%: FOR 5 MIN.

caA TING 90% 80-. (:0 AND 10% Mo on THE aLA/wr HEAT TREATING THE BLANKIN PURE N2 AT lsoac rap so m/v.

. MELT/N6 POINT OF N6 FIG 3 cn/r/cAL HEAT TREATMENT TEuPERATuRE-COMPLEM- NSAPPEARANCE on N016 MEL TING POIN T or p COMPOUND (8:20 mo)MEL mva POINT a; o couPa um: (40 wao) "I200 DISSOC/ATION or 28160 -5120FORMATION or 25 co Baa I000 IN l EN 7' 0/? J. MORRISON, JR.

A TIORNEV Patented Mar. 17, 1953 UNITED STATES PATENT OFFICE COLDCATHODE AND METHOD OF PREPARING SAME 6 Claims. (Cl. 117=-33.26)

This invention relates to electron emissive cathodes and moreparticularly to the preparation of activated surfaces for cold cathodesespecially suitable for use in gaseous discharge devices.

Gaseous discharge devices of the cold cathode type are often used inswitching operations, as in telephone circuits. In such circuits theyare mainly unattended and must have a long operating life expectancy.Prior coatings on'the cathode of such devices have greatly limited thelife expectancy of the device. These prior coatings, generally ofalkaline earth, for example barium and strontium, oxides on a nickelcathode blank, were necessarily too thin for long life expectancy. Thethin coating was required by the activation process which comprisedapplication of a series of condenser discharges through the device withpeak currents of many amperes. During this process, tiny arcs play overthe cathode surface converting the appearance and organization of thesurface. However, the activation will not proceed smoothly if thecoating thickness is greater than about 1 milligram per squarecentimeter. Since this activation process dislodged particles of thecoating and during the course of life the coating is slowly sputteredoff, the life,

of course, being dependent upon the amount of coating present, themethod of activation prevented attainment of a long life cathode.

Not only is a long life required of such devices but in most operationsa low sustaining voltage is. prerequisite. Thus, in many uses asustaining voltage of below '70 volt is required. The sustaining voltageof the final coating is dependent both on the composition of the coatingand on the temperature, and hence technique, of activation.

It is an object of this invention to provide an improved cathodecoating.

It is another object of this invention to provide a thick surfacecoating.

It is a, further object of this invention to provide such a coatinghaving a long life and a low sustaining voltage.

In accordance with a feature of this invention, the cold cathode surfaceconsists essentially of a mixture of 90 per cent barium carbonate and 10per cent nickelous oxide by weight sprayed on a nickel surface and heattreated at about 1300 C.

oil the composition percentages and particularly by'increasing thepercentage of nickel. It is a featureof this invention, however, thatparticular percentages of barium carbonate and-nickelous oxidewhenheated to particular critical high temperatures produce a cathodecoating havinga calculated operating life of forty years and asustaining voltage of less than volts, which critically hightemperatureruns counter to the general trend and teaching of the art.

It is a feature of this invention that an initial thick coating of percent barium carbonate and 10 per cent nickelous oxide be placed on anickel blank and heat treated above 1300 C.

It is a part of the discovery of this invention that when nickelousoxide and barium carbonate are heated, various compounds and eutecticsare formed,

It is thus a further feature of this invention that the heat treatmentbe carried on so that an eutectic of barium carbonate and barium oxidebe formed during the heat treatment and then be dissociated.

It is thus a further feature of this invention that the temperature ofthe heat treatment be such as to cause compounds formed in the BaO-NiOsystem to disappear, the temperature being about 1300 C.

It is a still further feature of this invention that the activatedcathode surface be of barium oxide with about 2 per cent of very finelydispersed nickel, the surface having a velvety brown coloration.

A complete understanding of this invention and of the various featuresthereof may be gained from consideration of the following detaileddescription and the accompanying drawing, in which:

Fig. 1 shows a gaseous discharge device employing the cathode coating ofthis invention;

Fig. 2 lists the steps in the process of preparing the cathode surface;and

Fig. 3 is a temperature diagram illustrating the changes in the cathodesurface during the heat treatment activation.

Referring now to the drawing,Fig. 1 shows one cold cathode gaseousdischarge device wherein the coating of this invention may be utilized.A cathode ll having the activating coating [2 of this invention thereonis supported by two leads [3 and [4 in an evacuated glass envelope l5. A

wire anode l6 having a ceramic insulating cylinder l1 around its lowerportions is positioned opposite the cathode II.

The cathode II is prepared in accordance with the steps outlined in Fig.2. A nickel cathode blank is degreased by rinsing in trichloroethyleneand then heat treated in wet hydrogen for fifminutes.

teen minutes at 1200 C. The heat treatment is to clean and degrease thesurface since the trichloroethylene rinse has been found to be notwholly effective. The back of the cathode blank is then renderedinactive so that there will be no glow discharge on this part of thecathode, as by being sprayed with a suspension of aluminum in amylacetate solution, though other glow deterrents may be employed. Thecalorized blanks are then heat treated in dry hydrogen for fifteenminutes at 925 C. and this treatment sinters and oxidizes the aluminumon the nickel, there being a suificient impurity of oxygen present in ofBa() alone would not be observed since its the system to oxidize thereadily oxidizable aluminum. The blanks themselves are then oxidized byheating for five minutes at 700 degrees in a mixture of 95 per centnitrogen, per cent oxygen in order to increase the ability of the fusedcoating to wet the surface. It has also been found that this lightoxidation is a sensitive and efiicient method for indicating thepresence of surface impurities. The cathode blanks are now ready for theapplication of the coating material.

The coating consists essentially of a mixture of 90 per cent bariumcarbonate and per cent nickelous oxide by weight sprayed on the nickelsurface using pyroxalin in amyl acetate solution as a vehicle. In onespecific embodiment, the nickelous oxide used is passed through a 400mesh sieve before mixing with the barium carbonate. In this particularembodiment, the mixture of barium carbonate-nickelous oxide in amylacetat-e solution was sprayed on the prepared cathode blank by an airbrush to a coating weight of 3.3 milligrams per square centimeter.

The cathode blank with this surface is then processed by heat treatmentin pure nitrogen for thirty minutes at substantially 1300 C. Thefurnace, in one specific technique, may be at 700 C. with purifiednitrogen flowing therethrough when the cathode blank is introduced. Thetemperature is then raised in about twenty minutes to 1300 C. where itis held for thirty The cathode is allowed to cool in the furnace untilthe temperature is below 600 C. when it may be removed. After removal itshould be kept in an evacuated container until ready for use in adevice, such as shown in Fig. 1. It has been found very advantageous inthis processing that pure nitrogen be employed as any oxygen impurity inthe gas will tend to fuse the coating to the extent that it will becomemolten and run off the nickel base.

The cathode surface following this heat treatment at 1300 C. will be inan active state. Following its incorporation in a device such as shownin Fig. 1, the cathode surface is brought to a uniform state ofactivation by drawing a higher than normal current density for a periodof one to two hours. The normal or operating current density of the tubewill depend on the geometry of the tube and the activating current maybe twice this Value.

Referring now to Fig. 3, the coating material has been found to undergoseveral changes from the time it is placed in the furnace tube at 700 C.to the end of its half hour treatment at 1300 C., which have definitebearing on the determination of 1300 C. as the critical temperature forthe optimum results with the specifically determined percentages ofinitial compounds. There is a low temperature melting observed at 1000'C. to 1050 C. which is, it is believed, due to the formation of aeutectic with the probable commelting point is 1923 C. It has been foundthat there are two compounds which are formed at temperatures above thedissociation of the 2BaCOs.BaO eutectic and which cause this secondstage of melting. A compound 3BaO.NiO, called a, has a melting point atabout 1200 degrees and a compound BaOtNiO', called ,8, has a meltingpoint at about 1250 degrees.

Partial fusion of the mixture of these compounds takes place and this isfollowed by gradual solidification of the mixture since in pure nitrogenat 1300 C. the N10 is reduced. It is thus apparent that it is essentialthat the temperature of the heat treatment he raised to about 1300degrees in order to remove traces of these undesirable compounds. 7

In order to attain the high temperature heat treatment at 1300 C., it isnot essential that the eutectic between BaCO3 and Eat) be formed at 1000C. Thus if the temperature of the heat treatment were first brought to950 C. and maintained there for a sufiicient period of time, such asfive or ten minutes, there would be a complete decomposition of thecarbonate in the nitrogen atmosphere. Further heating into the 1000 C.through 1050 C. range would produce no signs of melting nor formation ofthis eutectic. However, it has been found highly advantageous that thetemperature of the heat treatment be raised to about 1300 C. in suchsteps as will insure the formation and subsequent disappearance of the2BaCO3.BaO eutectic.

This heat treatment is carried on entirely at atmospheric pressures, towhich these melting points refer. The nickel blank itself, it is to beremembered, will melt at 1455 C. It has been found that the blank isdeleteriously affected when the temperature is raised to 1400" C.Therefore for these particular percentages and compounds, which arenecessary for the multiple beneficial results obtained, the range ofpossible temperatures of heat treatment is about 1300 C. to 1400 C. Ithas been found however, that for optimum results treatment at 1300 C..is most advantageous.

It is believed that the various compounds formed during the processingof the cathode blank are important for the preparation of the activesurface. Thus the fusion is believed to be necessary for the properincorporation of the nickel as an impurity in the BaO lattice, resultingin a coating of higher electrical conductivity. Therefore the heattreatment must be carried beyond such temperatures and in such a manneras will insure the formation of the compounds. However, while it is apart of the discovery of this invention that these compounds haveperformed a beneficial role in the changes occurring in the orientation,composition and lattice structure of the coating, the heat treatmentmust further be carried out in such a manner and in such a way as todissociate and remove them.

Cathodes prepared in the manner of this invention have been tested inorder to determine the final constituents. Within the limits of X-rayanalysis, the final surface is indistinguishable from pure BaO'.Chemical analysis, however, shows that there is still 1 to 3 per cent ofnickel in the active coating. The coating itself has a modifies theelectrical properties of the BaO so as to give an active cathodesurface.

These cathodes prepared in the manner of this invention may havecoatings of any desired thickness and still retain the otheradvantageous features of the activated coating and the low sustainingvoltage. Certain exemplary cathodes have been prepared with finalcoatings having thicknesses but little below the initial coating of 3.3milligrams per square centimeter, and a sustaining voltage of below 62volts, which values are illustrative.

The choice of the base material is important, as having an integral partin the processing and the nickel blank, prepared in the manner specifiedabove, has been found to be most advanvantageous for this purpose,

The cathode surface, as mentioned above, undergoes a partial fusionduring this process. This fusion produces a dense surface material onthe cathodes of velvety brown appearance. In the absence of this fusion,as when the heat treatment temperature is below 1300 C., the surface ofthe coating is very porous with poor adherence to the oxidized nickelbase. The surface hydrates readily, turning to a grey or in extremecases a white surface which will drop oif the base material. If toogreat a fusion takes place, as by having too high an oxygen content inthe nitrogen atmosphere, the surface will have a black metallicappearance. Under these conditions the coating passes through a fluidstate and a large portion of the coating runs off the nickel surface.

While various modifications may be made by those skilled in the artwithout departing from the spirit and scope of the invention, certainaspects are important to the preparation of an activated cathode surfacehaving the properties of a long life, a thick surface, a uniformlydistributed and partially fused surface and a low sustaining voltage.Thus to produce the resultant surface of this invention it is importantthat the initial composition be per cent nickelous oxide and 90 per centbarium carbonate and that the coating applied to the cathode blank beheated to at least substantially 1300 C., and advantageously not morethan 1400 C., but it is to be understood that the other above-describedarrangements are merely illustrative of the application of theprinciples of the invention.

What is claimed is:

1. An electron emissive cathode comprising a nickel blank and a thickelectron emissive velvety brown surface thereon, said surface consistingessentially of substantially 98 per cent barium oxide and 2 per centnickel in solid solution in the barium oxide lattice and having asustaining 7 Voltage of less than 70 volts.

2. The method of preparing an electron emissive cathode for glowdischarge devices which .6 comprises coating amixturerof 79031181 081;barium carbonate and 10 percent nickelous oxide on a nickel cathodeblank, heat treating said blank in an atmosphere free of oxygen and atatmospheric pressure to first form a BaCOsBaO eutectic and then todissociate said eutectic, continuing said "heat treatment to form aplurality of compounds of nickelous oxide and barium oxide, and thenmaintaining said heat treatment at a temperature between 1300 C. and1400 C., whereby said compounds are converted to barium oxide and finelydispersed nickel.

3. The method of preparing an electron emissive cathode for glowdischarge devices which comprises coating a mixture of per cent bariumcarbonate and 10 per cent nickelous oxide on a nickel cathode blank, andheat treating said blank in a nitrogen atmosphere at atmosphericpressure, said treatment comprising heating said blank to 1050 C. toform a BaCOaBaO eutectic, raising the temperature to dissociate saideutectic, raising the temperature to 1300 C. and maintaining the heattreatment at 1300 C. for 30 minutes to convert the 3BaO.NiO and BaO.NiOcompounds formed during said heating to barium oxide and finelydispersed nickel.

4. The method of preparing an electron emissive cathode for glowdischarge devices which comprises cleaning nickel cathode blanks,oxidizing said blanks in per cent nitrogen and 5 per cent oxygen for 5minutes at 700 0., coating said blank with 90 per cent barium carbonateand 10 per cent nickelous oxide, then placing said blanks in a furnaceat 700 0., raising the temperature of said furnace in steps to form andthen dissociate a BaCO3.BaO eutectic and then to 1300" C., andmaintaining said furnace at 1300 C. for 30 minutes, the heating beingdone in an atmosphere of nitrogen at atmospheric pressure.

5. The method of preparing an electron emissive cathode for glowdischarge devices comprising coating a mixture of 90 per cent bariumcarbonate and 10 per cent nickelous oxide onto a nickel cathode blankand heat treating said blank in an atmosphere free of oxygen and atatmospheric pressure, the heat treatment comprising raising thetemperature of said blank in steps to a temperature sufiicient to formcompounds of the BaO.NiO group, raising said temperature to about 1200C. to cause melting of a first one of said compounds, raising thetemperature to about 1250 C. to cause melting of a second one of saidcompounds, and then maintaining the temperature at between 1300' C. and1400 C. for a time sufiicient to assure complete conversion of saidcompounds to barium oxide and finely dispersed nickel.

6. The method of preparing an electron emissive cathode for glowdischarge devices which comprises cleaning a nickel cathode blank,oxidizing said blank in a slightly oxidizing atmosphere, coating saidblank with 3.3 milligrams per square centimeter of a mixture of 90 percent barium carbonate and 10 per cent nickelrius oxide, raising thetemperature of said blank in an atmosphere free of oxygen and atatmospheric pressure to a temperature sufficient to form compounds ofthe BaO.NiO group, raising said temperature to about 1200 C. to causemelting of the compound 3BaO.NiO, raising the temperature to about 1205C. to cause melting of the compound BaO.NiO, and then maintaining thetemperature at between 1300 C. and 1400" C.

r for a. time sufflcient to assure complete conversion dispersed nickel.

JAB/[ES MORRISON, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

Number '8 UNITED STATES PATENTS Name Date Bartlett et a1. June 9, 1931Pearcy Nov. 20, 1934 Wilson et a1 May 26, 1936 Hamada et a1 July 28,1936 Prescott Jan. 3, 1939 Umbreit July 30, 1940 Spencer Aug. 17, 1948

2. THE METHOD OF PREPARING AN ELECTRON EMISSIVE CATHODE FOR GLOWDISCHARGE DEVICES WHICH COMPRISES COATING A MIXTURE OF 90 PER CENTBARIUM CARBONATE AND 10 PER CENT NICKELOUS OXIDE ON A NICKEL CATHODEBLANK, HEAT TREATING SAID BLANK IN AN ATMOSPHERE FREE OF OXYGEN AND ATATMOSPHERIC PRESSURE TO FIRST FORM A BACO3BAO EUTECTIC AND THEN TODISSOCIATE SAID EUTECTIC, CONTINUING SAID HEAT TREATMENT TO FORM APLURALITY OF COMPOUNDS OF NICKELOUS OXIDE AND BARIUM OXIDE, AND THENMAINTAINING SAID HEAT TREATMENT AT A TEMPERATURE BETWEEN 1300* C. AND1400* C., WHEREBY SAID COMPOUNDS ARE CONVERTED TO BARIUM OXIDE ANDFINELY DISPERSED NICKEL.